Time of flight (TOF) mass spectrometers are widely used to determine the mass to charge ratio of charged particles on the basis of their flight time along a path. The charged particles, usually ions, are emitted from a pulsed source in the form of a packet, and are directed along a prescribed flight path through an evacuated space to impinge upon or pass through a detector. (Herein ions will be used as an example of charged particles.) In its simplest form, the path follows a straight line and in this case ions leaving the source with a constant kinetic energy reach the detector after a time which depends upon their mass to charge ratio, more massive ions being slower. The difference in flight times between ions of different mass-to-charge ratio depends upon the length of the flight path, amongst other things; longer flight paths increasing the time difference, which leads to an increase in mass resolution. When high mass resolution is required it is therefore desirable to increase the flight path length. However, increases in a simple linear path length lead to an enlarged instrument size, increasing manufacturing cost and require more laboratory space to house the instrument.
Various solutions have been proposed to increase the path length whilst maintaining a practical instrument size, by utilising more complex flight paths. Many examples of charged particle mirrors or reflectors have been described, as have electric and magnetic sectors, some examples of which are given by H. Wollnik and M. Przewloka in the Journal of Mass Spectrometry and Ion Processes, 96 (1990) 267-274, and G. Weiss in U.S. Pat. No. 6,828,553. In some cases two opposing reflectors or mirrors direct charged particles repeatedly back and forth between the reflectors or mirrors; offset reflectors or mirrors cause ions to follow a folded path; sectors direct ions around in a ring or a figure of “8” racetrack. Herein the terms reflector and mirror are used interchangeably and both refer to ion mirrors or ion reflectors unless otherwise stated. Many such configurations have been studied and will be known to those skilled in the art.
Mass selectors are well known in the art and are usually used for selecting ions of a small range of mass to charge ratios (m/z), often of a single m/z, for further processing. Quadrupole, magnetic sector and ion trap mass analysers are the most commonly used mass selectors. Ions having a wide range of m/z are typically emitted from an ion source and mass spectra are complex. Furthermore there may be several possible molecular candidates for any given ion. As is well known, in order to elucidate the molecular structure of an ion species, the species in question (the parent ion) is often subjected to fragmentation and the fragment ions are mass analysed, in a process termed MS-MS. The mass to charge ratios of the ions from the fragmentation process are characteristic of the parent ion. It greatly aids the identification process if the parent ion alone is subjected to the fragmentation process, and this often requires high mass resolving power (RP) to select the parent ions before passing them to the fragmentor.
TOF mass analysers are ideally suited to separate ions of high mass to charge ratios and to transmit a separated train of ions to a detection system or to additional ion optical devices for further processing. However, conventional TOF analysers suffer from high ion losses and poor focusing of ions. A small TOF analyser will have only modest mass RP and yet may still require very high speed switching hardware to enable ions of a relatively large range of mass to charge ratios (m/z) to be selected by time of flight gating structures. Such small TOF analysers may be inadequate for selecting ions of a single m/z. An example of such mass selector is shown in WO97048120. Where high RP selection is required, typically a costly and bulky TOF would be required. Other types of mass selector such as linear quadrupole mass filters are typically employed instead, but they have limited mass RP and, where relatively high mass RP filters are used, they have relatively low transmission (typically, when required mass windows are below 0.1-0.2 a.m.u.). Magnetic sector mass selectors extend the mass RP available over that possible from quadrupole mass filters, but magnetic sectors are very bulky, massive and costly. Both quadrupole mass filters and magnetic sector mass analysers have limited upper mass range. TOF mass analysers have the potential to be used as mass selectors with largely unlimited upper mass range and high mass RP, but so far transmission has been relatively low, and as already mentioned, the analysers are bulky and costly.
There remains a need for a high mass RP, high transmission, wide mass range, compact and reduced cost mass selector. Against this background, the present invention has been made.